Posted
by
timothyon Thursday September 08, 2011 @04:04PM
from the those-are-some-chatty-rocks dept.

sciencehabit writes "Gold, platinum, and other precioius metals were sucked into Earth's molten iron core soon after our planet formed. So where did all of the material for our fancy jewelry come from? According to high-precision measurements of two isotopes, or atomic variants, of tungsten in 4-billion-old rocks from Greenland published online today in Nature [the abstract adds a bit more; the full version is paywalled, though], precious-metal-bearing meteorites struck Earth around this time, coating the planet in a veneer containing gold, platinum, and other elements long after their native counterparts had disappeared into the planet's core."

Just a nitpick, but they use the word "veneer" several times in the abstract. It makes it sound like a thin solid sheet of precious metal. That's not what they're trying to imply. They're trying to emphasize the "thinness" of it, but not really getting the "scattered" part.

Probably "dusting" has some specific connotation to geologists. Maybe "scattering" would suit the situation.

You're right that it would have started out as a "dusting" or "scattering", but once you melt the crust and upper mantle, mix it up, and the whole thing starts cooling down and solidifying, it would be more like a "veneer" of comparatively enriched rock (enriched compared to what you would expect). But you're also right that it isn't a thin sheet of metal, we're talking about trace amounts distributed in the rock and which get subsequently concentrated into ore deposits mainly by transporting gold in solut

I've seen this so many times recently -- "Ha ha, Greenland is made of ice, and Iceland is made of green grass!" That's such an oversimplification. Iceland has the largest glacier in Europe; just ignoring all of Iceland's other glaciers, that one (Vatnajökull) alone takes up 8% of the country, and is very visible when you approach the country from the south and east (where most settlers would be coming from). And as for the "grass" part that sometimes gets thrown in when describing Iceland, it's not

Iceland was significantly more green at the time of settlement though - before it got deforested and the consequent erosion of most of the topsoil. The interior always was desert, iirc. A magnificent place, by the way.

True, over 1/3rd of Iceland used to be forested before human settlement. Well, as much as you can call Icelandic forests "forests";) But as you note, the interior was always desert, plus the glaciers were still giant rivers of ice back then, they still got their winter snows, etc. A funny thing, now with the introduction of the lupine/lúpína, some places in Iceland that were never able to be colonized before due to too hostile of a climate, like some of the sands in the northeast, are now bein

I've read the abstract but it's not clear that they're talking about enormous quantities of added gold/platinum/whatever. One interesting thing about gold, silver, copper, platinum, and some of the other precious metals is that they're soluble in hot water, so what you form is these huge underground plumes of rising hot water, over local hot magma areas, and the plumes are filled with dissolved metals. When the water rises enough it cools and the metals precipitate out -- primarily in cracks through which the water moves, forming veins that contain very high concentrations of precious metals. These plumes can be many, many miles high, and can pull up/concentrate metals from significant depths, so it's not clear to me that early gravity sorting of heavy metals downwards would result in no heavy metals at the surface. (An interesting side-note is that since each metal has a different solubility in water, as the water rises and cools, different metals precipitate out at different points, so if you find silver you're likely to find at least some gold nearby, but most likely not at exactly the same spot.) Note that I'm not a geologist, just an amateur gold hunter, but this is the explanation I've been given by my geologist friends.

Not to mention that asteroids can cause sizeable ore deposits without bearing the ore themselves. A good example is the Creighton nickel/copper/platinum-group metal deposit under Sudbury, Ontario. The area was struck by a huge asteroid 1.85B years ago, which blasted out a large chunk of the crust. The huge transient crater (250km across -- contrast with 170km for Chicxulub) then became filled with a mantle melt plume, yielding large amounts of heavier metals close to the surface.

Of course all the heavier elements came from the star that was around here before our Sun and solar system formed, but that doesn't mean what we find on the surface all came from meteorites and asteroids after the planet's formation.

I've read the abstract too, but having been reading Moorbath et al's work for years, studying the chemical evolution of the mantle, I've got a better idea what they're talking about.

Your comments about the non-zero solubility of [anything] in hot water are not incorrect, but not relevant to this work.

The LHB veneer idea doesn't claim that there would be no siderophile elements at the Earth's surface after segregation of the core. It simply states that the early mantle and the early segregating core would have been in (approximate) chemical equilibrium, and consequently the concentrations of different elements in the iron-rich phase compared to the silicate-rich phase would have approximated towards the concentrations predicted from the partition coefficients of the relevant species in the relevant conditions. (This is a tautology - that is what "partition coefficient" means ; whether the conditions approached equilibrium is a more moot point, but they would probably have satisfied my Mantle Petrology tutor's criteria of being to more than 100km depth for more than 100 million years, which is a regime not terribly amenable to experiment.)

However, when one does the sums, and plugs in the experimental data that one has, one finds that there is, in most mantle samples from 3+ billion years ago and 100+km below the surface, more of various elements, including tungsten, gold, PGEs, etc., which should have been taken core-wards with the differentiation of the planet. So, either the segregation process and the rates of diffusion were less efficient than we have reason to believe, or there is something peculiar going on.

It gets more peculiar though... some sources of samples (diamonds, to be precise) do show mantle materials that have been depleted in siderophile elements to the expected extent (compared to meteorite materials).

So in some places the process works as expected, and in others, it doesn't. Which is damned peculiar. And that peculiarity is what makes them come up with the model of late accretion of a modest amount of (undepleted chondritic) material onto the upper surface of the mantle after the accumulation of 90%+ of the Earth and the segregation of the core.

That puts it into the time scale appropriate for the (probable) Moon-forming Giant Impact, and for the considerably later "Late Heavy Bombardment", but I've not yet seen anyone explicitly linking the 3 events into two or even one event.

Memo to self : must make time to attend the next public lecture I hear of being given by Moorbath; this is a dead-interesting topic.

While I'm sure that some precious metals have arrived from space after the earth's accretion, I think it's rather a stretch to think that the concentration in places like South Dakota or Alaska/Yukon are the result of impact events.

It's true that these elements settled into lower strata, but it must also be remembered that many volcanic events are fueled by plumes of material that emanate from the core itself (the Yellowstone Caldera is believed to be such). That certain metals were concentrated in the lower strata during Earth's early formation does not mean that 100% of them stayed there.

While I'm sure that some precious metals have arrived from space after the earth's accretion, I think it's rather a stretch to think that the concentration in places like South Dakota or Alaska/Yukon are the result of impact events.

It's true that these elements settled into lower strata, but it must also be remembered that many volcanic events are fueled by plumes of material that emanate from the core itself (the Yellowstone Caldera is believed to be such). That certain metals were concentrated in the lower strata during Earth's early formation does not mean that 100% of them stayed there.

I think there is a slight misunderstanding here. Even the so-called mantle "superplumes" only originate at the core-mantle boundary; it is not generally considered (afaik) that they actually involve any significant amount of core material. The density difference between the liquid outer core and the lower mantle is too great for significant amounts of the core to join an upwelling through the less-dense mantle, even if significantly hotter. These plumes are generally considered to be made up of mantle mater

There are two things being neglected in that, a) scale and b) the difference between (relatively) recent geological events and those that would have occurred in the immediate aftermath of the formation of a stable crust.

As significant as large regional veins of precious metals' ore are to mining operations, if one hypothesizes that they were originally brought to the outer crust by superplume-driven volcanic events that would make Krakatoa look like a science fair display they would account for a very, ve

I would like to also point out that at any time magma can touch these formations deep within the core through shifts in the tectonic plates, thereby completely melting any deposit of precious ore. Where does this magma bring these materials in liquid form, probably where most people are really thinking of,such as the upper depths and not the lower ones......or the lower ones...bringing them to the next shifted region.....etc....

I also think at some point if a crater was created by a striking meteor, contain

In fact, it wouldn't make a speck of sense if it was Iceland being studied, because Iceland is a geologically very young volcanic island with rocks no more than ~40 million years old, whereas the rocks being studied in this paper are 4 billion years or so and among the oldest on Earth. The whole point of the paper is to show that tungsten isotopes have changed over Earth history, and that the change happened quite early. They do compare the old Greenland tungsten isotopic measurements to more recent igneous rocks such as the ones from Iceland, but you could have as easily mentioned Hawaii, the Azores, the Canary Islands, and several other "recent" locations used for the comparison. Iceland isn't special in that respect.

The premise of this paper is that the difference can be explained if the early Earth (>4 billion or so) chemically differentiated initially and most of the siderophile elements (things like tungsten, gold, platinum, etc.) sank to the core during that process, leaving the surface rocks more depleted. That's the time the Greenland samples may represent. Then at a younger time, speculated to be near the ~3.8 billion year late heavy bombardment [wikipedia.org], a bunch more meteoritic stuff was dumped on the top (more siderophile-enriched), mixed into the upper part of the mantle, and igneous rocks have been generated mainly from that upper mantle source ever since (including the more modern samples they are comparing to, and also the ~2 billion-year-old samples they also show). There are other scenarios, but it is plausible and ties in with other evidence about the late heavy bombardment (such as Nd isotopic data from Sm/Nd and Hf/W dating). They model the effects of some alternative models and show those models can't easily be used to explain what is seen. It's a pretty testable hypothesis as people continue to do tungsten isotope studies on rocks of a variety of ages before and after the late heavy bombardment. This is a pretty bold paper.

"I don't understand. Why should tungsten isotopic abundances change over time on Earth as compared to in space?"

Do you *really* want me to try to explain that?:-) Well, hey, this is slashdot. People here aren't stupid and I'm up for a challenge, although I'm not an expert on this stuff either.

Okay, first of all there are several isotopes of tungsten (W). The ones relevant in this paper are 182W and 184W. These are both stable, non-radioactive isotopes, but the 182W is also produced by the decay of radi

My degree is in geology; while I have no problem with the idea that some of the deposits came from asteroids and the like, there are far too many other ways that many of these deposits can be formed here on earth. I know that for precious metals like gold and silver, hydro-thermal deposits are quite common sources of these ores (with a large number of these being found in or around granite sources.)

My degree is in geology; while I have no problem with the idea that some of the deposits came from asteroids and the like, there are far too many other ways that many of these deposits can be formed here on earth. I know that for precious metals like gold and silver, hydro-thermal deposits are quite common sources of these ores (with a large number of these being found in or around granite sources.)

The question isn't really about where the deposits near the surface come from; these are almost always the product of circulation of water/rock through the crust and upper mantle, as you suggest. The question is why these very dense minerals are available in the upper mantle/crust to be deposited in the first place. Going by density alone, one would expect that the same processes that resulted in an iron/nickel core during early stages of the formation of the Earth would result in the majority of precious m

It sounds like any time geologists can't figure something out, the answer is "meteors". From life, to water, to now gold and silver, the refrain is "meteors". Kinda starting to sound a bit repetitive over the past decade, or is it just me? I am obviously not a geologist.

How did that Icelandic rock (mentioned in title) get to Greenland (summary)? Is the submitter perhaps afflicted with geographasia americana, whose symptoms include thinking that distinct countries/provinces are the same thing?

Forget Earth--go to the Moon or Mars for getting the goodies closer to or at the core. Sure, both still have molten gooey centers but we can get a lot closer to the center than here on Earth. After all, if one says meteorites brought metals to Earth the same deal applies to the Moon especially since it used to be part of Earth. (No doubt stirring the pot when that happened...)

Heck, digging deep enough to get heat and power from the core would make it much less likely for a stray meteorite to wipe out your l

... precious-metal-bearing meteorites struck Earth around this time, coating the planet in a veneer containing gold, platinum, and other elements long after their native counterparts had disappeared into the planet's core."

Correct me if I'm wrong (I'm sure you will), but isn't the present theory of the origin of the Earth that it was formed by meteoritic bombardment? Where would these so-called "native counterparts" have come from except from said meteors? This summary suggests all the glittery stuff came from the stragglers of the same event.

Isn't the Earth considered to be about 4 billion years old? In other words, WTF is the summary suggesting (besides a monumental lack of reading comprehension)?!?

What are "4-billion-old rocks" [sic].

[I won't bother to mention the Greenland/Iceland confusion as others are carrying that torch well already.]

A collective faceplant rang out like a thunderclap as all the under-employed geeks wondered how people like that get and retain employment.

Here's an idea. Maybe/. editors should run their stuff by the other/. editors before unleashing their stuff upon the general readership. Consider it something like a code review, or quality assurance, or basic best practice.

Or, is this a cleverly devised feature of/. to pump up comments? If so, I don't think clever really comes into it.

Well then, if a significant amount of gold came from meteorite bombardment. Then doesn't it hold that our local asteroid fields should have vast amounts of the stuff? Shouldn't a spectral scan of the sky hint at all this?

I don't think you realize how expensive it is to leave the gravity well. Even if there were solid gold bullion in orbit there would have to be a fuckton of it just for the operation to break even.

Mining in space will be important, but it's unlikely to be important or cost effective to get the materials back to Earth. Whatever is ultimately mined in space will likely stay in space to build things there.

Mining in space will be important, but it's unlikely to be important or cost effective to get the materials back to Earth. Whatever is ultimately mined in space will likely stay in space to build things there.

Not so. It's easy to get things back to Earth once you get out there. Sure, it's very expensive to get there, but so long as you're prepared to stay for a long time you can amortize over the mission.

The delta-V you'd need to hit a 3-5 years delivery window from the asteroid belt to Earth would be pre

That seems logical. The earth and the other rocky planets' native heavy metals sank to their respective cores during formation. The asteroid belt being either the result of the breakup of a planet or its failure to form would consist of deposits of these materials randomly distributed throughout the belt.

the denser metals such as Cu, Pt, Ni, Ag, and Au should have started to stratify over 4B years. Therefore, what we need is a really big straw, about 4000mi/6400km long to stick into the earth's core and pump out all the those valuable metals. The straw will probably need to be made of graphite and/or carbon nanotubes to handle the heat. On the plus side, it may be a diamond when we remove it.

Think of the side benefits. We can pump iron and transuranic wastes in to replace what we're drawing out. The transur

Rock acts differently under the kinds of pressures you get when you get close to the mantle.

From what I understand, it can flow to some degree, even in solid form. Thus, your well keeps trying to collapse while you're drilling. Also, with the high pressures come high temperatures, and it's difficult to drill in high temperature situations (this is what shut down the Kola Borehole in Russia).

Drilling is also really expensive, and it gets more so the deeper you go. You need special (read: expensive) materi

"Really big straw" is a fair description of deep well drilling, and there are a lot of (uninformed) people who think drilling into the mantle can be done using current generation technology. The government has assigned grants for projects that would lay the groundwork for your straw.

So while your post may have been satirical, it wasn't really all that farfetched.

I'm familiar with DSDP, ODP, IODP, and Kola. And yes, drilling into rock at 180c-300c requires some engineering. But we have cutting tools that handle much higher temperatures than that. The bottom line is that we've drilled only a little deeper (about 900m) than the Challenger Deep in the Mariani Trench, the lowest known place on earth. Kola only made it about 1/3 of the estimated distance through the crust.